8th Annual Symposium on Signal Integrity PENN STATE, Harrisburg Center for Signal Integrity
Practical Measurements of Dielectric Constant and Loss for PCB Materials at High Frequency Practical Measurements of Dielectric Constant and Loss for PCB Materials at High Frequency
Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
Common Test Methods for Material Electrical Characterization
Circuit Evaluation Techniques for Material Characterization Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
Wavelength (λ) is the physical length from one point of a wave to the same point on the next wave Long wavelength = low frequency and the opposite is true Short wavelength = more waves in the same time frame so higher frequency Amplitude is the height of the wave and often related to power High electric field = High magnetic field = High amplitude = High power Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board) • Transverse ElectroMagnetic (TEM) wave Electric field varies in z axis Magnetic field varies in x axis Wave propagation is in y axis • TEM wave propagation is most common in PCB technology, but there are other waves Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
Other wave propagation modes are: TE (transverse-Electric) or H wave; magnetic field travels along with wave TM (transverse-Magnetic) or E wave; electric field travels along with wave TEM or quasi TEM waves are typically the intended wave for a transmission line Some PCB design scenarios will have problems with “modes” or “moding” Moding issues are when the intended TEM wave is interfered with another wave mode such as TE or TM modes; this is a spurious parasitic wave or unwanted wave Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
• When an EM wave transitions from free space to a medium of higher
relative permittivity (εr or dielectric constant or Dk) it will:
• have slower velocity • have a shorter wavelength • and the amplitude is reduced
Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
Circuit with low Dk Circuit with high Dk Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
Resonators used in PCB technology are often based on ½ wavelength
Feed line Resonator element Feed line
gap Top view of gap coupled resonator gap
The resonator element has the physical length of ½ wavelength for the 1st resonant frequency node
Basically a standing wave is established and a lot of energy is generated at the “resonant” frequency Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
Relative permittivity defined, by electric field and dipole moments
D = εE D is electric displacement vector, E is electric field intensity, ε is complex permittivity
• When an electric field is applied to a dielectric material, electric dipole moments are created
• The dipole moments augment the total displacement flux
• Additional polarization (P) is due to the material and its’ related dipole moments
D = ε0E + P
ε0 is free space permittivity Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
Relative permittivity defined, by electric field and dipole moments
• Dielectrics used in the high frequency PCB industry are typically a “linear dielectric”
• Or P is linear with an applied E so:
P = ε0 c E
c is electric susceptibility of the material Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
Relative permittivity defined, by electric field and dipole moments
• Finally, the displacement flux, including material effects:
D = ε0E + P = ε0 (1+ c) E = εE
ε = ε’ – jε” = ε0 (1 + c)
• ε’ is the real (storage) and ε” is the imaginary (dissipative)
• ε’ is associated with dielectric constant and ε” is associated with dissipation factor (Df) of the material
Dk = εr = ε’/ε0
Df = Tan() = ε”/ε’ Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
Relative permittivity defined, by electric field and dipole moments
• From about 100 MHz to 300 GHz most interaction between electric fields and the substrate material is due to displacement and rotation of the dipoles
• The dipole displacement contributes to the Dk (εr)
• Molecular friction due to dipole rotation contributes to tan() or Df
Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
• Dispersion is how much the Dk will change with a change in frequency
• Dipole moment relaxation is another issue which contributes to dispersion
• At low frequencies the dipole relaxation has little affect on Dk dispersion
• At microwave frequencies dipole relaxation has more affect on dispersion Basic ElectroMagnetic Concepts for Frequency vs. Dk curve for a PCB (Printed Circuit Board) generic dielectric material
Low loss materials have much less Dk-Frequency slope
1 GHz 10 GHz 100 GHz
Dipolar and related relaxation phenomena Dk
Df
All circuit materials have dispersion (Dk changes with frequency) Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board) Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board)
PCB Losses
αT is total insertion loss • Insertion loss is the total loss of a high frequency PCB αC is conductor loss α is dielectric loss • There are 4 components of insertion loss D αR is radiation loss αL is leakage loss
• Typically RF leakage loss is considered insignificant for PCB, but there are exceptions • Microwave engineering puts a lot of emphasis on conductor and dielectric loss • mmWave engineering focuses on conductor, dielectric and radiation loss
Microwave is 300 MHz to 30 GHz Millimeter-wave (mmWave) is 30 GHz to 300 GHz Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board) PCB Losses Dielectric Losses Attenuation (reduction) of the signal energy due to the substrate Mostly due to the Tan or dissipation factor (Df) of the substrate
A rougher surface is a longer path for a wave to propagate. Conductor Losses Conductor losses are due to several factors: Besides the resistance of the copper, due to skin effects, it may be the copper treatment Copper surface roughness that is used. DC and AC resistance of the conductor Ground return path resistance The ground return path narrows with higher Skin effects frequency. Less copper area used, so more resistance. Permeability of the conductor This is unusual but some metal finish or copper treatment have ferromagnetic properties with increased loss due to the equivalent of high Df in regards to permeability Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board) PCB Losses
• There are many variables regarding radiation loss • Radiation loss is: frequency radiation loss • Frequency dependent • Circuit thickness dependent thickness radiation loss • Dielectric constant (Dk) dependent Dk radiation loss • Radiation loss can vary intensity due to: • Circuit configuration (microstrip, coplanar, stripline) • Signal launch • Spurious wave mode propagation • Impedance transitions and discontinuities Basic ElectroMagnetic Concepts for PCB (Printed Circuit Board) PCB Losses
Different components of loss in regards to thickness for a microstrip PCB
Dissecting losses when using the same material at different thickness for microstrip TL Common Test Methods for Material Electrical Characterization
• IPC has 13 different test methods to determine Dk and / or Df • ASTM and NIST have several test methods • Many OEM’s and Universities have their own test methods • Each test method has its own pro's and con's • The results of one test may not correlate well to the results of another method, when using the exact same material • There is No Perfect test method Common Test Methods for Material Electrical Characterization
Common material test methods:
Full Sheet Resonance (FSR) test
Clamped Stripline Resonator test
Split Post Dielectric Resonator (SPDR) test
Split Cylinder Resonator test
Rectangular Waveguide resonator test Common Test Methods for Material Electrical Characterization
FSRFull Sheet Resonance (FSR) test, IPC-TM-650 2.5.5.6
Network Analyzer sweeps a range of frequencies and evaluates at what frequency there are standing waves or resonant peaks Knowing the exact length of the panel, and the resonant frequency peak the Dk is calculated Common Test Methods for Material Electrical Characterization
FSRFull Sheet Resonance (FSR) test, IPC-TM-650 2.5.5.6
• The panel is acting like a parallel plate waveguide • FSR can only determine Dk and not Df • This is because we can not accurately account for radiation loss • The open sides of the panel allow radiation losses Common Test Methods for Material Electrical Characterization
FSRFull Sheet Resonance (FSR) test, IPC-TM-650 2.5.5.6
Isolated nodes over short range of frequencies Node 1,0 Node 2,0 Node 2,2
Multiple nodes (resonant peaks) over wider range of frequencies
Length axis nodes only Both axes nodes Common Test Methods for Material Electrical Characterization
Full Sheet Resonance (FSR) test, IPC-TM-650 2.5.5.6
A “node” is based on the number of ½ wavelengths in a direction on the panel
Node 1,0 is 1 half wavelength in the length direction and No wave in the width
Node 1,2 is 1 half wavelength in the length direction and 2 half wavelengths in the width direction (not shown)
Node 1,0
Node 2,0
Side view of the panel under test in the length axis Common Test Methods for Material Electrical Characterization
Full Sheet Resonance (FSR) test, IPC-TM-650 2.5.5.6
Wave Interference patterns Constructive: When two waves collide of the same wavelength and at the same phase angle, the resultant wave has a significantly increased amplitude (shown)
Destructive: When two waves collide of the same wavelength and are 180 degrees out of phase (1/2 wavelength), both waves are nullified (not shown)
Example of Constructive Interference shown Common Test Methods for Material Electrical Characterization
Full Sheet Resonance (FSR) test, IPC-TM-650 2.5.5.6
For a rectangular panel it is best to Node 1,0 Node 2,0 Node 2,2 Node 3,0 measure nodes 1,0 and 2,0
These nodes are in the range of frequency where only the length axis has standing waves
The nodes above 2,0 can have interference due to wave propagating in both axes
Example: node 3,0 can have interference due to the other waves near its frequency. It can be seen that node 3,0 is not a well defined peak as nodes 1,0 and 2,0. Length axis nodes only Both axes nodes Common Test Methods for Material Electrical Characterization
Full Sheet Resonance (FSR) test, IPC-TM-650 2.5.5.6
•Pro's • Quick and simple test • Accurate determination of Dk • Minimal operator dependencies • Non-destructive test •Con's • Can not test for dissipation factor • Thin materials may have Dk accuracy concerns • Measurements are at a lower frequency (typ. < 1 GHz) Common Test Methods for Material Electrical Characterization
X-Band Clamped Stripline Resonator test, IPC-TM-650 2.5.5.5c • Raw material is clamped together with resonator card in between • The outside metal clamps act as the ground planes for the stripline
Top view of resonator card
Stripline resonator Stripline resonator Side view of resonator card clamped into test fixture Common Test Methods for Material Electrical Characterization
X-Band Clamped Stripline Resonator test, IPC-TM-650 2.5.5.5c
• We test at 10 GHz, per IPC, but since the resonator will resonate at 1/2 wavelengths, some other frequencies can be tested • What can be tested accurately, with our default equipment is: 2.5 GHz testing with 1/2 a wavelength • 2.5 GHz or node 1 • 5.0 GHz • 7.5 GHz • 10.0 GHz • 12.5 GHz • Any frequency above this we would need to change the cables, fixture and connectors that we use
10 GHz testing with four ½ wavelengths, 4 half wavelengths or node 4 Common Test Methods for Material Electrical Characterization
X-Band Clamped Stripline Resonator test, IPC-TM-650 2.5.5.5c Node 4, 10 GHz Common Test Methods for Material Electrical Characterization
X-Band Clamped Stripline Resonator test, IPC-TM-650 2.5.5.5c
• There is some amount of entrapped air • Certain materials with rougher surface will have more air entrapped
• The entrapped air will cause the test to report Side view of resonator card clamped into test fixture a lower Dk
Resonator element • Material with a high degree of anisotropy can accuracy concerns Gap coupling (2X)
Feed lines (2X) Common Test Methods for Material Electrical Characterization X-Band Clamped Stripline Resonator test, IPC-TM-650 2.5.5.5c
• Pro's: • Reports Dk and Df (no radiation losses) • Very good for a fast test, high frequency Dk / Df test • Simple structure allows simple calculations • Good accuracy for Dk and moderately good for Df • Minimal operator dependencies • Testing is done in the range of many user applications (2-10 GHz) • Con's: • Dk can be reported lower than actual circuits with some materials • Destructive test • Limited material configurations • Some resonator cards may change over time Common Test Methods for Material Electrical Characterization
Split Post Dielectric Resonator (SPDR) test
• A resonator that compares the baseline measurement of an empty cavity (air) to a cavity with material • There is an electric field established between the two resonators (top and bottom) • The associated wave pattern is a right hand circular polarized TE mode • The electrical properties of the material is evaluated in the x-y plane only
Rogers Proprietary Common Test Methods for Material Electrical Characterization
Split Post Dielectric Resonator (SPDR) test
• SPDR testing is sample thickness dependent • SPDR fixture that is tuned to 10 GHz can test material that is 12mils or less • SPDR tuned to 20 GHz can test material that is 25mils or less • There is no minimum thickness, in theory • Sample can not sag and it must remain planar with no bow or twist • A very accurate thickness measurement is critical for Dk and less critical for Df • Since it only evaluates materials in the x-y plane there can be significantly different Dk numbers of some materials compared to FSR and stripline testing Common Test Methods for Material Electrical Characterization
Split Post Dielectric Resonator (SPDR) test
• Pro's • Very fast and user friendly test • Assuming an accurate and repeatable thickness measurement method, then SPDR is accurate and repeatable • Can stack samples of different material in SPDR for evaluating composite Dk and Df • SPDR is sometimes used with FSR or clamped stripline to evaluate anisotropy • Con's • Doesn't test the z-axis • Glass reinforced or filled materials that are polarized will report significantly different Dk values compared to results from FSR and stripline test methods • Accuracy of the thickness measurement is extremely critical for Dk values Circuit Evaluation Techniques for Material Characterization
Microstrip transmission line testing
Microstrip gap coupled strip resonators
Microstrip ring resonators
Microstrip couplers
Microstrip 180° Hybrids
Microstrip stub tuning networks
Microstrip delay lines
Many of these circuits can use other circuit configurations such as grounded coplanar or stripline, however there are less circuit fabrication variables with non-pth microstrip Circuit Evaluation Techniques for Material Characterization
Microstrip differential phase length method, transmission line testing
Signal layer microstrip Ground layer Substrate = εr
Cross-sectional view
Uses microstrip transmission line circuits of different length; typically 3:1 length ratio
Circuits are:
• identical in everywhere except for length • are made in very near proximity of each other on the same panel • 50 ohm characteristic impedance
RO4003CTM laminate
Rogers Confidential 38 Circuit Evaluation Techniques for Material Characterization
Microstrip differential phase length method, transmission line testing
Measurements are taken of the phase angle at a specific frequency for each circuit. The microstrip phase angle formula is used and altered to accommodate two circuits of different length:
eff (Φ) phase angle for single circuit of 2f L length (L) at a specific frequency (f) c
(ΔΦ) difference of phase angle for two 2f eff L circuits at a specific frequency (f) with c a difference of circuit length (ΔL)
2 c Formula rearranged to solve for eff 2fL effective dielectric constant (εeff)
Once εeff is solved, MWI-2010 or a EM field solver is used to calculate the Dk of the material at that specific frequency. This procedure is repeated by increasing to the next frequency
and recalculating the εeff and solving for the Dk. Circuit Evaluation Techniques for Material Characterization
Microstrip differential phase length method, transmission line testing
• Example of data collected for the 2” transmission line circuits, for frequency (Hz) vs. Unwrapped Phase angle shown the left; saved in *.prn format. • Once the *.prn file with the frequency-phase data for both the 2” and 6” circuit is read into MWI-2010 and the details of the circuit construction are entered then the software outputs a *.txt file which can be read into Excel. Freq. (GHz) Effective Dk Dk Circuit Evaluation Techniques for Material Characterization
Microstrip differential phase length method, transmission line testing Circuit Evaluation Techniques for Material Characterization
Microstrip differential phase length method, transmission line testing
• Pro's • Copper surface roughness affects are captured • Copper surface roughness has an impact on the phase constant Allen Horn, III*, John Reynolds*, and James Rautio+; *Rogers Corporation, +Sonnet software, “Conductor Profile Effects on the Propagation Constant of Microstrip Transmission Lines, IEEE MTT-S, 2010. • Wideband Dk vs. Frequency data • Results are from actual circuit testing and not a fixture or raw material sampling • Con's • Time consuming to design, make circuits and evaluate them • This method is a transmission / reflection technique which is typically not as accurate as a resonator technique • Wideband signal launch can be an issue • Wideband mode suppression can be an issue Circuit Evaluation Techniques for Material Characterization
Microstrip differential length method, transmission line insertion loss testing
• This method uses the same principle as the Differential Phase Length method • Except this method is using the S21 magnitude values from the short and long circuits • The same pressure contact connectors are used and oriented to the same ports during testing • The loss of the short circuit is subtracted from the long circuit, leaving loss as dB/unit_length • The subtraction of the loss of the two circuits is intended to eliminate the loss of the connectors and the signal launch Circuit Evaluation Techniques for Material Characterization
Microstrip differential length method, transmission line insertion loss testing
Screen shots from PNA while testing two circuits of the same material which are different length only
2” microstrip transmission line 6” microstrip transmission line
Circuit material used is 10mil thick RO4350BTM laminate Circuit Evaluation Techniques for Material Characterization
Microstrip differential length method, transmission line insertion loss testing
Insertion loss results: Circuit Evaluation Techniques for Material Characterization
Microstrip differential length method, transmission line insertion loss testing
• Pro's • Copper surface roughness affects are captured • Copper surface roughness has an impact on insertion loss J. W. Reynolds, P. A. LaFrance, J. C. Rautio, A. F. Horn III, “Effect of conductor profile on the insertion loss, propagation constant, and dispersion in thin high frequency transmission lines,” DesignCon 2010. • Wideband Insertion loss vs. Frequency data • Results are from actual circuit testing and not a fixture or raw material sampling • Con's • Time consuming to design, make circuits and evaluate them • Wideband signal launch can be an issue • Wideband mode suppression can be an issue Circuit Evaluation Techniques for Material Characterization
Side note: Microstrip transmission line testing to obtain Df (dissipation factor)
• Some companies will use microstrip transmission line testing to back calculate the Df • Typically the Df of the material is not accurately found from transmission line testing • Many times the reported Df has the conductor loss included as wells as radiation loss • It is recommended not to extrapolate Df from transmission line S21 measurements due to many variables which impact the accuracy: • To calculate the Df, the conductor loss and radiation loss must be subtracted • Conductor loss is affected by copper surface roughness • The impact of copper surface roughness on loss is frequency dependent • There are many different methods for calculating surface roughness affect on conductor loss and each method has its own set of limits and capabilities • Radiation loss can be difficult to accurately account due to the wideband measurements as well as differences in signal launch impacting radiation loss • Varying levels of return loss or mismatch loss may not be well captured • Df calculation is better done on resonant structures than transmission / reflection Circuit Evaluation Techniques for Material Characterization Microstrip gap coupled strip resonators and ring resonators • Gap coupled strip resonators are used to evaluate materials for Dk and Df
• These structures do have some amount of radiation loss
• Sometimes they are tested in a grounded metal enclosure to capture the radiation losses
• The gap coupling should be loosely coupled to realize the Q of the dielectric more than the conductor Q
• The gap coupling can affect the center frequency and cause inaccuracies in determining Dk and Df
Feed line Resonator element Feed line
Gap (ΔL) Top view of gap coupled resonator Gap (ΔL) Circuit Evaluation Techniques for Material Characterization Microstrip gap coupled strip resonators and ring resonators
• A method was developed to eliminate the potential impact of the gaps
• Again, a differential length method is used
Eq. For long resonator
Eq. For short resonator
Simultaneously solve ΔL is the added length of the to eliminate ΔL resonator due to fringing and is dependent on the gap size Circuit Evaluation Techniques for Material Characterization Microstrip gap coupled strip resonators and ring resonators
• Taking the differential length method of resonators to the next step was to use ring resonators
• Ring resonators, when designed correctly, have minimal or no radiation loss
• The gap coupling can impact the resonant frequency and the calculations of Dk and Df
• Using the previous method, the impact of the gaps can be minimized
• Ring resonators can be designed with the exact same feed line, gaps and other dimensions, with the only difference being the circumference
• The two circumferences needs to be a multiple of common resonant nodes Circuit Evaluation Techniques for Material Characterization Microstrip gap coupled strip resonators and ring resonators
example using a 1 GHz and 5 GHz ring resonators built on 5mil RO3003
Screen shot of Excel worksheet for the ring resonator nodes at 25 GHz
Below are screen shots from the PNA for the ring resonators at 25 GHz
1 GHz ring 5 GHz ring node 50 node 10 Circuit Evaluation Techniques for Material Characterization Microstrip gap coupled strip resonators and ring resonators Circuit Evaluation Techniques for Material Characterization
Microstrip differential circumference ring resonator testing
• Pro's • Ring resonators have minimal or no radiation loss so calculated Df can be more accurate • There is more freedom in designing the gap coupling so it will not impact the accuracy of the calculated Dk values • The is a lot of literature and references for using ring resonators regarding material characterization • Results are narrowband; less issue with signal launch and spurious modes • Con's • Time consuming to design, make circuits and evaluate them • Results are narrowband and limited information for wideband applications